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The receiver consists of eight data lanes plus one forwarded-clock lane supporting the hyper transport standard for high-density chip-to-chip links. This all-digital clock and data recovery (ADCDR) circuit, which is well suited for today?s CMOS process scaling, enables the receiver to achieve low power and area consumption. The ADCDR can enter into open loop after lock-in to save power and avoid clock dithering phenomenon. Moreover, to compensate the open loop, a phase tracking procedure is proposed to enable the ADCDR to track the phase drift due to the voltage and temperature variations. Furthermore, the all-digital delay-locked loop circuit integrated in the ADCDR can generate accurate multiphase clocks with the proposed calibrated locking algorithm in the presence of process variations.
HIGH-DENSITY forwarded-clock (FC) links have been widely used in the multi-core processor?s interfaces for the chip-to-chip interconnects, such as Quick-Path Interconnect (QPI), hyper transport, and DDR standards. A dedicated clock is delivered from the transmitter to receiver and shared by multiple data lanes. Although the additional clock lane consumes extra pins, area, and power, all of these can be amortized among many data lanes. The clock and data recovery (CDR) circuit in the FC receiver is used to align the received clock with the data for error-free sampling and it has become the most critical and power/area-hungry component in receiver.
Ring-oscillator-based calibration: This system gives to guarantee accurate phase relationship in the presence of process variations using separated delay code for each sub-delay line. Unfortunately, the ring oscillator formed by the sub-delay line during calibration has to run at a multiplying frequency of the reference clock.
Difficult to satisfy the timing constraint in high-speed applications and become much worse when more multi-phase clocks are demanded.
Calibration method: A simpler and effective calibration method is proposed to eliminate process mismatch with negligible penalties of the area, power, and performance. To track the phase shift due to the voltage and temperature variations, all-digital DLLs/CDRs need continue working in close loop after lock-in. However, the clock phase will remain being adjusted according to the closed-loop early/late feedbacks and moving backward and forward, which is called clock dithering.
Design: All-digital DLL-based clock generator